1. Field of the Invention
The present invention relates to a method of hydroforming of a hollow structural component such as, for example, an outer tube portion of a muffler of a vehicle.
2. Description of Background Art
See, for example, Japanese Patent No. 3654210.
In the method disclosed in Japanese Patent No. 3654210, a joined plate member, formed by superimposing plates on each other and joining their outer peripheral portions together, is placed between a pair of female dies having recessed spaces formed therein respectively and being arranged with open sides of the recessed spaces facing each other. A fluid such as, for example, water is poured into the joined plate member from an inlet opening opened at a predetermined portion of the joined plate member. By the fluid pressure of the fluid, the joined plate member is pressed outwardly. Thereby, the joined plate member is shaped into a hollow structural component having a shape corresponding to the recessed spaces.
In the method according to Japanese Patent No. 3654210, a perimeter portion of the joined plate member, placed between outer peripheral portions of the recessed portions of the pair of female dies (this portion is called a blank portion hereinbelow), is secured by being pressed between the outer peripheral portions of the recessed portions of the pair of female dies.
However, when the blank portion is pressed like this, a frictional force, that acts when the joined plate member, is drawn to the recessed spaces of the female dies by the fluid pressure varies according to the shape of the recessed spaces of the female dies. Thus, the joined plate member might locally experience an excessive tension force, making the plate thickness of the hollow structural component uneven.